For both XML and HTML, the document can provide an encoding
either in XMLDecl in XML, or as a meta element in HTML head.
This adds options to ignore those encodings if the encoding
is known in advace for example if the content had been converted
before being passed to the parser.

* parser.c include/libxml/parser.h: add XML_PARSE_IGNORE_ENC option
  for XML parsing
* include/libxml/HTMLparser.h HTMLparser.c: adds the
  HTML_PARSE_IGNORE_ENC for HTML parsing
* HTMLtree.c: fix the handling of saving when an unknown encoding is
  defined in meta document header
* xmllint.c: add a --noenc option to activate the new parser options